Reflection 139: Horseshoe Crab Consciousness

August 7, 2009

By June, 2005, I got pretty good at thinking like a horseshoe crab. I’d been tracking 26 of them in Taunton Bay for two years, and was finally able to anticipate their movements with some success. In doing his biological assessment of the bay, biologist Slade Moore had suggested a cooperative horseshoe crab tracking program with Friends of Taunton Bay. Slade worked out of the West Boothbay offices of the Maine Department of Marine Resources, and I was to do most of the tracking. In June of 2003, we epoxied miniature sonar transmitters on 26 crabs, 13 in Hog Bay, 13 in Egypt Bay. My job was to track the movements of each transmitter with a sonar receiver and hydrophone from a small boat. If the epoxy held and the transmitter stayed in place, the location of a transmitter would indicate the location of the crab we’d attached it to.

We tested the range at which we could detect a sonar signal, and based on the assumption the bottom of the bay was flat, Slade worked out a grid of listening stations where we would turn the hydrophone 360 degrees, and record the direction in which we heard any signals. That way, we could triangulate a crab’s location if we picked up its signal at two or more stations. Which seemed like a reasonable plan, until we got a great many signals bouncing off underwater boulders and ledges, and determined the bottom was full of ridges and depressions that effectively blocked reception of even nearby signals. Our backup plan was to steer the boat as close as we could come to being directly on top of a signal—when it would be loudest in our headphones, and use a GIS receiver to log the boat’s—and presumably the crab’s—position. It took a while to work the bugs out of the tracking program, but on good days, both Slade and I were able to pinpoint the location of most transmitters in both bays. On other days, we might find only half of them.

What does this have to do with consciousness? For starters, we were engaged in a project never attempted in Taunton Bay, so we were relying on on-the-job experience to train ourselves in the use of a new language we invented as we went, somewhat as a child creates language by observing the sounds people make on certain occasions, and then mimics them. Too, we were not only interested in where a given crab was, but wanted to know what it was doing and why it was there. Ultimately, we wanted to know what factors governed horseshoe crab movements at different times of year. Factors like water temperature, water depth, habitat type, hormone levels and mating urges, food availability (such as small clams, worms, mussels), presence of predators, and so on. The big questions were where and when did the range of local horseshoe crabs overlap with beds of blue mussels, so that dragging for mussels might put them at risk?

Like every other small embayment, Taunton Bay exhibits a range of features that make it unique. Because of its extensive system of mudflats, it has more mussels, clams, and marine worms than many other bays. Which attracts predators of mussels, clams, and worms—such as horseshoe crabs, flounders, ring-billed gulls, several duck species, and predators of predators such as harbor seals, striped killifish (which prey on horseshoe crab eggs), ospreys, and eagles. And human harvesters who go after any one of them by dragging, digging, pulling, fishing, or hunting.

Too, the upper reaches of Taunton Bay are extremely shallow, making them warmer in summer and colder in winter than many other bays in Maine. Every creature that lives on the flats has to adapt one way or another to a wide range of seasonal temperatures. Harbor seals, for instance, exit the bay in December when ice begins to form, leaving only the alpha male to defend his territory against rivals. A big question for the horseshoe crab tracking program was where do members of the two local sub-populations spend the winter? When we started the program, we fully expected local crabs to seek deeper, warmer waters in the Gulf of Maine during the winter, and to return to mate in warm shallows in late spring. We were planning to install a fixed hydrophone aimed across Taunton River to record their departure and return. We never deployed that second hydrophone because, as we found in November 2003, the horseshoe crabs of Taunton Bay retire from shallow flats to the upper slopes of deeper channels, where they bury themselves in the mud and wait out the winter by suppressing their food intake, breathing, and blood flow, becoming as close to inanimate as creatures can get while still retaining the ability to reanimate themselves when conditions improve six months later.

That was big learning for us because by rights, horseshoe crabs have no place in a northern bay that freezes-over in winter. Throughout most of their global range—including the Indian Ocean, southwestern Pacific, and Atlantic as far south as the Yucatan Peninsula—they move to deeper waters in winter and remain active for the duration. Now it appears that in several bays in Maine and New Hampshire, they hibernate, slowing their metabolisms to the survivable limit for six months of the year. In our study at the northern limit of their global range in Taunton Bay, we found them hunkering down and not moving again until the third week in April, when they’d take a month to feed and work their way upslope to their breeding shores, where females would lay their eggs and males fertilize them in the shallow sand and gravel nests where they were deposited. In the narrow confines of horseshoe crab research, that was a breakthrough—a mind-expanding discovery.

Imagine sleeping for half a year, waking up, eating a big breakfast, then looking to have a year’s worth of sex in the two weeks you’ve got before having to store up enough calories to get you through the following winter! Horseshoe crabs in Taunton Bay live like that, not day-by-day or month-by-month but year-by-year.

Another thing we found was that horseshoe crabs do not adopt routine ways of meeting their needs but reinvent themselves almost constantly to see if, hit or miss, they can’t find a way to adapt to their surroundings wherever they are. No wonder they’ve been around for more than four-hundred million years. There are only a few shores around the bay suitable for digging nests in sand or small gravel. The bay is largely ringed by outcrops of bedrock, with a few stony beaches interspersed here and there. Of those beaches, many are armored with cobbles and boulders left by the Laurentian Ice Sheet that retreated 12,000 years ago. Most shores provide only a few patches here and there of substrates suitable for digging nests and laying eggs. Horseshoe crabs seem to find breeding sites almost at random, females constantly testing the bottom in their travels, digging down when they hit the occasional soft spot. If it’s too rocky or rooty, they are thwarted and move on, always testing as they go, scratching, scratching, scratching.

Which is pretty much how they select a mate. A male horseshoe crab will try any stone, log, or available boot in searching for a mate. If female hormones are in the water, that doesn’t mean your average male knows what secreted them. Males apparently can tell how close they are to the source by the concentration of the scent, but any smooth rounded shape is worth a try until they find something they can clasp onto. Then they hold tight and won’t let go, unless perhaps flipped upside down by a wave, or trapped in a too-narrow passage between adjacent rocks—and even then they tend to cling to the death.

Taunton Bay crabs seem to find suitable places to hibernate almost at random, with no fidelity to that nice little B&B they found last year. They live out their lives within less than a mile of suitable breeding shores, but during winter settle in on the upper margin of any channel within that range. Usually among mussels or eelgrass. They can crush small shellfish and eat them, so they seem to hibernate in areas where mussels are plentiful. Mussels thrive near eelgrass beds on the upper slopes of channels where tidal currents provide phytoplankton through the seasons. Horseshoe crabs wander around in search of food until they stumble across a bed of mussels, which makes their decision for them. In trying to think like a horseshoe crab, I found myself thinking like a mussel as well, or the rare patch of sandy gravel—both of which the local horseshoe crab life cycle depends on.

The big challenge was locating horseshoe crabs I couldn’t see solely by the sonar signals their affixed transmitters emitted. The bottom is nowhere near level, it turns out—even where flats looks level at low tide. I’d consult my GPS unit and go back to where I found one the day before, put down the hydrophone, and hear nothing but white noise. Each signal was coded with an identifying frequency and sequence of sounds, so if we heard any signal at all we could tell which transmitter was the source. But silence told us nothing but that nobody was home. The challenge was always: which way did they go? As I grew more familiar with horseshoe crab ways, and the bottom terrain in their respective sub-embayments, I began to grasp more of how they might relate to a given site. And then imagine which way they would head from there. Following crude hunches upslope or down, along the edges of channels one way or another, I found I could recover signals far more readily than simply heading off on any random heading as I had done as a novice tracker. I didn’t really think like a horseshoe crab because I don’t have any idea what a horseshoe crab would think even if it could. But I developed a sense of where they would be heading at that time of year, and I let that sense steer my boat, often to a successful encounter.

When they’d rouse from their winter sleep in late April, horseshoe crabs would head for the nearest food supply in the vicinity. I got pretty good at predicting where that would be. Then they’d head upslope to breeding shores, and I got so I could follow along. In late May and June, I’d start out near known nesting shores. In July and August, they could be almost anywhere, but usually in shallower water. In September and October, they’d head downslope toward the network of channels, usually where eelgrass or mussels were concentrated. And in November they’d select their hibernation site, usually in the middle of a plentiful food supply. Several things could go wrong with that simple scheme. Transmitters could stop sending signals, or become detached and send false signals. Horseshoe crabs aren’t supposed to shed their shells after reaching sexual maturity at age 10—but maybe they do now and then. I got several signals that never moved from week to week, even in mid-summer. Slade managed to retrieve one transmitter from the mud and re-epoxy it to another crab. In any project things are going to go wrong, and you just have to roll with the punches.

Tracking horseshoe crabs changed my consciousness for life, much as living with a pet will expand your awareness to include other ways of looking at the world. It isn’t the owner who walks the favored pet so much as the pet who takes its “owner” on daily walks. I certainly was led on many a boat trip by horseshoe crabs, and I know the underwater terrain of Taunton Bay much better as a result. I hold mussels, clams, eelgrass, horseshoe crabs, and striped killifish in mind as living beings, not objects of casual curiosity. To know about a species in a general sense, you have to befriend it and meet its members up-close and personal in the most particular and detailed way possible. Knowledge obtained from books or the Internet cannot affect you as deeply as knowledge gained through firsthand experience.

Thinking like a horseshoe crab requires reading backwards from observable behaviors to recreating the “mind” responsible for those behaviors. A module of my brain is now dedicated to doing just that with horseshoe crabs at the northern limit of their range on Earth. Other modules are devoted to eelgrass, ring-billed gulls, bald eagles, even slime molds. As my friend Anette Axtmann once said, “We have much to learn from wood lilies.” To be as truly knowledgeable as we need to be in this time of estrangement from life on Earth, we have much to learn from the plants and animals who share our native habitat. Which requires us to take the initiative in giving our minds to them so that we pay attention to what matters on Earth and not solely what concerns us at the moment. The future depends on us expanding our consciousness to include other such beings in order to live with them on equal terms, not on our forcing them to adapt to our ways as if we were in charge.

To care for the Earth, we must first become mindful of its creatures and their ways, making room for them in our consciousness so that we can incorporate them into our thinking, and more importantly, our acting on the world stage. It is no accident that mussel draggers keep away from horseshoe crab habitats in Taunton Bay. The Taunton Bay Advisory Group incorporated horseshoe crabs in its thinking, and then into its recommendations for regulating local fisheries. When horseshoe crabs speak, we do our best to get their message—and act accordingly.